The invention relates to a shift device of an epicyclic change-speed gear transmission having a forward gear clutch for at least one forward gear to provide a forward drive and a reverse gear clutch and a reverse gear brake to control a reverse gear for a rearward drive, which clutches and brake are engaged by associated shift control means connected respectively to a switch valve means and blocking means to inhibit the engagement of at least one of the reverse gear clutch and reverse gear brake when the forward gear clutch is engaged with a blocking control means responsive to a blocking pressure, whereby the associated shift valve means is lockable in a position disengaging the reverse gear brake.
In a shift device of this type ("Automatische MB-Getriebe Typ W3D 080/R und W3A 110/R" [Automatic MB gears type W3D 080/R and W3A 110/R"] introductory document of Daimler-Benz Aktiengesellschaft KD 20 200 112200-877 4, 2 pages 005/23 to 25), a shift valve of a reverse gear brake is actuable into its position, disconnecting the work pressure by a regulator pressure used as blocking pressure as a function of the travelling speed. This work pressure is used to simultaneously stress the shift control element of the reverse gear clutch, and is also switched off by a manually actuable selector slide valve when the latter is not in its position for a reverse gear drive. The shift valve of the forward gear clutch is likewise supplied with work pressure from a selector slide valve when the latter occupies one of the positions associated with the forward gears. This work pressure maintains the shift valve in its position engaging the forward gear clutch. When the selector slide valve is moved into the position for the reverse gear drive, the pressure line leading to the shift valve of the forward gear clutch is relieved of pressure by the selector slide valve so that the forward gear clutch disengages when the reverse gear brake and reverse gear clutch engage and vice versa.
An underlying aim of the instant invention is to substantially prevent the possibility of the reverse gear being shifted accidentally during the time a shifted forward gear is engaged. These accidental shifts cannot be excluded in the case of shift devices in which, contrary to known shift devices, have neither a manually actuable selector slide valve nor a regulator pressure set as a function of the travel speed, but rather, have shift valves changed over by a control pressure impulse which is determined by solenoid control valves modulated by an electronic control unit. Due to unavoidable leakages in aluminum housings, which accommodates the steel shift slide valves of the shift devices, it is possible for a pressure to build up in the control pressure line leading from the associated solenoid control valve to the shift valves of the reverse gear while the forward gear clutch is engaged, which leads to a shifting engagement of the reverse gear during forward gear engagement.
In order to avoid such dual engagement of forward and reverse gears, there is provided an epicyclic change-speed gear transmission having a forward gear clutch for at least one forward gear to provide a forward drive and a reverse gear clutch and a reverse gear brake to control a reverse gear for a rearward drive, which clutches and brake are engaged by associated shift control means connected respectively to a switch valve means and blocking means to inhibit the engagement of at least one of the reverse gear clutch and reverse gear brake when the forward gear clutch is engaged with a blocking control means responsive to a blocking pressure, whereby the associated shift valve means is lockable in a position disengaging the reverse gear brake.
In the shift device according to the invention, when the forward gear clutch is engaged, its work pressure acts upon the shift valve of the reverse gear brake conjointly with an associated valve spring so that the shift valve is locked in its position maintaining the brake disengaged, so that accidental leakage pressures in the control pressure line, which flows oppositely to the shift valve, cannot lead to accidental changeovers. The unblocking of this shift valve in the course of a normally tripped shifting of the reverse gear may occur, in the case of a structurally separate arrangement of the shift valve of the forward gear clutch from the shift valve of the reverse gear brake, in that the latter is changed over by the control pressure of the solenoid control valve and the blocking pressure at the shift valve of the reverse gear brake is also switched off thereby.
However, if the shift valves of the forward gear clutch and of the reverse gear brake operate with a common shift slide valve wherein a blocking pressure of a blocking control element is connected to pressurize said common slide valve through a valve connection for a work pressure line leading to said shift control means of the forward gear clutch, and wherein said slide valve includes a slide duct leading at one end into said pressure blocking chamber and at another end into a valve housing chamber communicating openly with said valve connection leading to said shift control means of the forward gear clutch, then the unblocking of this common shift valve occurs in the course of a normal shifting of the reverse gear. This is true in that first of all, the shift valve of the reverse gear clutch changes over and connects the relevant shift control element to the system pressure line at a section thereof located between a throttle and shift valve which causes a pressure decrease through the connected control pressure pipe which switches on a receiver which affects the valve control element acting upon the connected regulator valve. By this means, the system pressure in the section of the system pressure line located between throttle and receiver is regulated to a lower pressure value. However, the common shift valve is also connected to this pipe section, so that the work pressure of the forward gear clutch and hence the blocking pressure at this common shift valve are reduced to this low pressure value, whereby the blocking is cancelled and the valve changes over.
This unblocking occurs with greater reliability in the shift device according to the invention if the control pressure controlled by the solenoid control valve means is higher than said regulated reduced pressure of the system pressure line.
It is also advantageous if the shift valve means of the reverse gear brake has a slide valve with one end thereof having a pressure chamber stressable by said control pressure and another end having a pressure chamber of a blocking control element stressable by said forward gear work pressure of the forward gear clutch, by which arrangement and construction of the locking control element, the shift device may be used regardless of whether the shift device operates with two separate or with one common shift valve for forward gear clutch and reverse gear brake.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for the purposes of illustration only, a single embodiment in accordance with the present invention, and wherein: